Leukotriene B4 (LTB4) is a key mediator of inflammatory processes, immune responses, and host defenses against infection (1-4). It stimulates chemotaxis, degranulation, release of lysosomal enzymes, and the production of reactive oxygen species (ROS) (5-7). In fact, LTB4 is one of the most potent chemoattractants known, acting mainly on granulocytes and monocytes (8, 9). Recently, it was also shown to be a chemoattractant for effector CD4+ and CD8+ T lymphocytes, recruiting them to sites of acute inflammation (10-15). It also promotes cell adhesion to vascular endothelial cells and transmigration (8, 16), which amplifies inflammatory early responses. Although LTB4-induced leukocyte recruitment is thought to play a protective role in host defense against various pathogens, it is also involved in a number of human inflammatory diseases such as asthma (17-20), a disease of chronic airway inflammation characterized by eosinophilic infiltration, mucus hypersecretion, and airway hyperresponsiveness (AHR). Thus, a significantly increased level of LTB4 is detected in the airways of patients with asthma and also in experimental models of asthma (20).
LTB4 produces its biological effects via specific G protein-coupled receptors known as BLT1 and BLT2 (21-24). To date, most studies of LTB4 receptors have focused on the high-affinity LTB4 receptor, BLT1, expressed exclusively in leukocytes, especially its role in inflammatory responses (22). For example, early recruitment of neutrophils and eosinophils into the airways in response to allergen inhalation is reduced in BLT1-deficient mice (8, 25), suggesting a role of BLT1 in the chemotaxis of granulocytes in allergic asthma. In addition, BLT1 is essential for the allergen-mediated early recruitment of CD4+ and CD8+ T cells into the lung airways and the development of allergen-induced AHR and inflammation under certain experimental conditions (26, 27). In contrast to BLT1, BLT2 has a low affinity for LTB4 and is expressed in a wide variety of tissues, with highest levels in the spleen, leukocytes and ovary (23).
The role of BLT2 in the pathogenesis of asthma was investigated using a murine model. By employing antisense to block endogenous BLT2 expression, a critical role for BLT2 in the development of AHR and airway inflammation was demonstrated. In addition, without being bound to a particular theory, BLT2 causes asthmatic symptoms by elevating ROS generation and subsequent NF-κB activation. Furthermore, immunohistochmical analysis of clinical asthma samples revealed a significant elevation of expression of BLT2 mainly in the airway epithelial layers as well as in the microvascular endothelium, which is similar to the pattern observed in the murine model of asthma.
Asthma is thought to be caused by a combination of genetic and environmental factors. The prevalence of asthma has increased significantly since the 1970s. As of 2010, 300 million people were affected worldwide. In 2009 asthma caused 250,000 deaths globally. Symptoms of asthma can be prevented by avoiding triggers, such as allergens and irritants, and by inhaling corticosteroids. However, long-term corticosteroid use has the potential to cause severe side-effects including, e.g., hyperglycemia, insulin resistance, diabetes mellitus, osteoporosis, cataract, anxiety, depression, colitis, hypertension, ictus, erectile dysfunction, hypogonadism, hypothyroidism, amenorrhoea, and retinopathy. Accordingly, new compositions and methods for treating asthma are urgently required.